Harry,
The benefit of connecting to a monitoring center is that you can spread the cost of an operator between yourself and tens or hundreds of the centers other clients. However, that means any given operator at the center is going to be responsible for far more cameras than they can effectively monitor. Usuallly, you will want to use event based monitoring to get the operators attention when something of interest is happening. Figuring out how to trigger these events is your first step. Common solutions are burglar alarms, video analytics, or simple motion detection (video or PIR). Once you have settled on an effective form of event detection, that helps you narrow the field based on who supports a given technology.
I have had success with VideoIQ video analytics and ViewpointCRM for monitoring - but that was for a fairly specific scenario. YMMV.